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<table class="t-dcl-begin"><tbody>
<tr class="t-dsc-header">
<td> <div>定义于头文件 <code>&lt;threads.h&gt;</code>
 </div></td>
<td></td>
<td></td>
</tr>
<tr class="t-dcl t-since-c11">
<td> <div><span class="mw-geshi c source-c"><span class="kw4">void</span> call_once<span class="br0">(</span> <a href="../thread.html"><span class="kw1022">once_flag</span></a><span class="sy2">*</span> flag, <span class="kw4">void</span> <span class="br0">(</span><span class="sy2">*</span>func<span class="br0">)</span><span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span> <span class="br0">)</span><span class="sy4">;</span></span></div></td>
<td> (1) </td>
<td> <span class="t-mark-rev t-since-c11">(C11 起)</span> </td>
</tr>
<tr class="t-dcl t-since-c11">
<td> <div><span class="mw-geshi c source-c"><span class="kw1">typedef</span> <span class="coMULTI">/* unspecified */</span> <a href="../thread.html"><span class="kw1022">once_flag</span></a></span></div></td>
<td> (2) </td>
<td> <span class="t-mark-rev t-since-c11">(C11 起)</span> </td>
</tr>
<tr class="t-dcl t-since-c11">
<td> <div><span class="mw-geshi c source-c"><span class="co2">#define ONCE_FLAG_INIT /* unspecified */</span></span></div></td>
<td> (3) </td>
<td> <span class="t-mark-rev t-since-c11">(C11 起)</span> </td>
</tr>
<tr class="t-dcl-sep"><td></td><td></td><td></td></tr>
</tbody></table>
<div class="t-li1"><span class="t-li">1)</span> 准确调用函数 <code>func</code> 一次，即使从多个线程调用。函数 <code>func</code> 的完成与先前或后继的用同一 <code>flag</code> 对象的对 <code>call_once</code> 调用同步。</div>
<div class="t-li1"><span class="t-li">2)</span> 足以保有 <code>call_once</code> 所用标志的完整对象类型。</div>
<div class="t-li1"><span class="t-li">3)</span> 展开成能用于初始化 <code>once_flag</code> 类型对象的值。</div>
<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
<table class="t-par-begin">


<tr class="t-par">
<td>  flag
</td>
<td> -
</td>
<td> 指向用于确保只调用一次 <code>func</code> 的 <code>call_once</code> 对象的指针
</td></tr>
<tr class="t-par">
<td>  func
</td>
<td> -
</td>
<td> 只执行一次的函数
</td></tr></table>
<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
<p>（无）
</p>
<h3><span class="mw-headline" id=".E6.B3.A8.E6.84.8F">注意</span></h3>
<p>此函数的 POSIX 等价物是 <a rel="nofollow" class="external text" href="http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_once.html">pthread_once</a> 。
</p>
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">运行此代码</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="co2">#include &lt;stdio.h&gt;</span>
<span class="co2">#include &lt;threads.h&gt;</span>
 
<span class="kw4">void</span> do_once<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span> <span class="br0">{</span>
    <a href="../io/puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"called once"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">static</span> <a href="../thread.html"><span class="kw1022">once_flag</span></a> flag <span class="sy1">=</span> <a href="ONCE_FLAG_INIT.html"><span class="kw1023">ONCE_FLAG_INIT</span></a><span class="sy4">;</span>
<span class="kw4">int</span> func<span class="br0">(</span><span class="kw4">void</span><span class="sy2">*</span> data<span class="br0">)</span>
<span class="br0">{</span>
    call_once<span class="br0">(</span><span class="sy3">&amp;</span>flag, do_once<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../thread.html"><span class="kw996">thrd_t</span></a> t1, t2, t3, t4<span class="sy4">;</span>
    <a href="thrd_create.html"><span class="kw997">thrd_create</span></a><span class="br0">(</span><span class="sy3">&amp;</span>t1, func, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
    <a href="thrd_create.html"><span class="kw997">thrd_create</span></a><span class="br0">(</span><span class="sy3">&amp;</span>t2, func, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
    <a href="thrd_create.html"><span class="kw997">thrd_create</span></a><span class="br0">(</span><span class="sy3">&amp;</span>t3, func, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
    <a href="thrd_create.html"><span class="kw997">thrd_create</span></a><span class="br0">(</span><span class="sy3">&amp;</span>t4, func, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
 
    <a href="thrd_join.html"><span class="kw1004">thrd_join</span></a><span class="br0">(</span>t1, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
    <a href="thrd_join.html"><span class="kw1004">thrd_join</span></a><span class="br0">(</span>t2, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
    <a href="thrd_join.html"><span class="kw1004">thrd_join</span></a><span class="br0">(</span>t3, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
    <a href="thrd_join.html"><span class="kw1004">thrd_join</span></a><span class="br0">(</span>t4, <a href="../types/NULL.html"><span class="kw103">NULL</span></a><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>输出：
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">called once</pre></div></div> 
</div>
<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
<div class="t-ref-std-11">
<ul><li> C11 standard (ISO/IEC 9899:2011): 
</li></ul>
<dl><dd><ul><li> 7.26.2.1 The call_once function (p: 378)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 7.26.1/3 ONCE_FLAG_INIT (p: 376)
</li></ul>
</dd></dl>
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<table class="t-dsc-begin">

</table>

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